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The US team, from Brown University in New England, made the find after putting 35 healthy men and women through sensitive brain scans as they slept. The scans were done twice - once on their first night asleep in a strange place, then a week later. They showed that during the first night, the left side of the brain did not shut down properly when sleep should be at its deepest.

The left side of the brain controls the right side of body and the volunteers were more likely to wake up if sounds were played into their right ear than their left. But when tested a week later, both sides, or hemispheres, of the brain shut down.

Writing in the journal Current Biology, the researchers said: "These lines of evidence are in accord with the hypothesis that troubled sleep in an unfamiliar environment is an act for survival over an unfamiliar and potentially dangerous environment by keeping one hemisphere partially more vigilant than the other as a night watch, which wakes the sleeper up when unfamiliar external signals are detected."

They said it was not clear why it was always the left side that stayed alert. However, they only monitored brain activity for an hour or two and it is possible that later in the night the right side took over.

Researcher Yuka Sasaki said: "Our brains may have a miniature system of what whales and dolphins have."